Semiconductor materials

Semiconductor materials are insulators at zero temperature that conduct electricity in a limited way at room temperature. The defining property of a semiconductor material is that it can be doped with impurities that alter its electronic properties in a controllable way.

Because of their application in devices like transistors (and therefore computers) and lasers, the search for new semiconductor materials and the improvement of existing materials is an important field of study in materials science.

The most commonly used semiconductor materials are crystalline inorganic solids. These materials can be classified according to the periodic table groups from which their constituent atoms come.

It is clear from the part of the Periodic Table of Elements shown below, that GaAs is one of a great number of possible binary III-V compounds, such as also GaP, InP, AlAs, GaN etc formed by combination of Group III elements and Group V elements at equal atomic concentrations. Ternary and quaternary III-V compounds, such as Al(x)Ga(1-x)As, InAs(1-y)P(y), In(x)Ga(1-x)As(1-y)P(y) can also be formed and many of them also have valuable properties for semiconductor technology. The II-VI semiconductors comprise the compounds containing Zn, Cd and Hg as the cations and O, S, Se and Te as the anions.

II	III	IV	V	VI
-	B	C	N	O
-	Al	Si	P	S
Zn	Ga	Ge	As	Se
Cd	In	Sn	Sb	Te
Hg	-	-	-	-

By far, silicon (Si) is the most widely used semiconductor material as of 2004. Its combination of low raw material cost, reasonable speed, relatively simple processing, and a useful temperature range make it currently the best compromise among the various competing materials. Silicon is currently fabricated into boules that are large enough to allow the production of 300mm (approximately 12 inch) wafers. The III-V and II-VI compounds comprising elements of lower atomic numbers are more ionic (ie, less covalent) than are the III-V and II-VI compounds of elements of higher atomic numbers. The usual result is that the lower-atomic-number semiconducting compounds have larger valence-band to conduction-band energy gaps E_g.

Germanium (Ge) was a widely used early semiconductor material but its lower melting point makes it less useful than silicon. Today, germanium is often alloyed with silicon for use in very-high-speed SiGe devices; IBM is a major producer of such devices.

Gallium arsenide (GaAs) is also widely used in high-speed devices but so far, it has been difficult to form large-diameter boules of this material, limiting the wafer diameter to a few hundred millimeters and making mass production of GaAs devices significantly more expensive than silicon.

Other less common materials are also in use or under investigation.

Silicon carbide (SiC) has found some application as the raw material for blue light emitting diodes (LEDs) and is being investigated for use in semiconductor devices that could withstand very high operating temperatures and environments with the presence of significant levels of ionizing radiation. IMPATT diodes have also been fabricated from SiC.

Various indium compounds (indium arsenide, indium antimonide, and indium phosphide) are also being investigated as is selenium sulfide.

Semiconductors with predictable, reliable electronic properties are difficult to mass-produce because of the required chemical purity, and the perfection of the crystal structure, which are needed to make devices. Because the presence of impurities in very small proportions can have such big effects on the properties of the material, the level of chemical purity needed is extremely high. Techniques for achieving such high purity include zone refining, in which part of a solid crystal is melted. Impurities tend to concentrate in the melted region, leaving the solid material more pure. A high degree of crystalline perfection is also required, since faults in crystal structure such as dislocations, twins, and stacking faults, create energy levels in the band gap, interfering with the electronic properties of the material. Faults like these are a major cause of defective devices in production processes. The larger the crystal, the harder it is to achieve the necessary purity and perfection; current mass production processes use six-inch diameter crystals which are grown as cylinders and sliced into wafers.